Drug-eluting implantable medical devices have become popular in recent times for their ability to perform their primary function (such as structural support) and their ability to medically treat the area in which they are implanted. Further, stents made from shape memory materials, particularly nitinol, have become popular.
Stents formed from nitinol include many characteristics desirable in an effective stent. Nitinol is a nickel-titanium alloy generally containing approximately 55-56% nickel and 44-45% titanium. Nitinol was developed by the Naval Ordinance Laboratory and receives its name from its component parts and the Naval Ordinance Laboratory (Nickel/Titanium/Naval Ordinance Laboratory). Specifically, stents formed from nitinol, with or without special coatings, have been found to be chemically and biologically inert and to inhibit thrombus formation. Nitinol, under certain conditions, is also superelastic, which allows it to withstand extensive deformation and still resume its original shape. Furthermore, nitinol possesses shape memory, i.e., the metal “remembers” a specific shape fixed during a particular heat treatment and can resort to that shape under proper conditions.
The superelasticity of nitinol and its shape memory characteristics makes it possible to fabricate a stent having the desired shape and dimensions. Once formed, the stent can be temporarily deformed into a much narrower shape for insertion into the body. Once in place, the stent can be made to resume its desired shape and dimensions. Certain alloys of nickel and titanium can be made which are plastic at temperatures below about 30° C. and are elastic at body temperatures above 35° C. Such alloys are widely used for the production of stents for medical use since these nitinol stents are able to resume their desired shape at normal body temperature without the need to artificially heat the stent
While using nitinol for stents is desirable, nitinol material presents some difficulties in the formation of the stent itself. Nitinol materials in either the cold worked or heat-treated state can be easily sheared or stamped, but they are difficult to form to an accurate geometry, whether by forming wire shapes or die pressing. Thus, many nitinol stents are formed from a nitinol tube that is laser cut to the shape of a stent, sometimes also known as a tubular slotted stent. However, many stents are formed by manipulating a wire into a desired stent shape. When forming such a stent from a nitinol wire, complicated or specific design fixtures are required to hold the nitinol wire in the desired pattern throughout the heat setting, or heat treatment, process cycle. Typical process steps when forming a nitinol wire to be used as a stent include: conforming the nitinol wire to the geometry of the fixture; placing the nitinol wire and fixture into a “furnace” or other heating device for a set temperature and duration; removing the nitinol wire and fixture from the heating device and quenching (flash cooling); and removing the nitinol wire from the fixture. Custom fixtures may be required for each particular stent design. It is also often difficult to generate a cost effective fixture for simple and complicated stent patterns. Simpler wire forming methods available for stents made from other materials, where controlled plastic deformation of the wire into the desired shape allows for the wire to hold its shape through further processing, are generally not available for use with nitinol wires. For example, and not by way of limitation, methods and devices for creating waveforms in a wire described in U.S. Application Publication Nos. 2010/0269950 to Hoff et al. and 2011/0070358 to Mauch et al., and co-pending U.S. application Ser. Nos. 13/191,134 and 13/190,775, filed Jul. 26, 2011, may not effectively be used to form nitinol wire stents.
Thus, there is a need for an improved method for forming a stent from a nitinol wire, and in particular, and improved method of forming a stent with a hollow nitinol wire.